Virus classification is the process of naming viruses and placing them into a taxonomic system. Similar to the classification systems used for cellular organisms, virus classification is the subject of ongoing debate and proposals. This is mainly due to the pseudo-living nature of viruses, which is to say they are non-living particles with some chemical characteristics similar to those of life. As such, they do not fit neatly into the established biological classification system in place for cellular organisms.
Contents
- Virus species definition
- ICTV classification
- Structure based virus classification
- Baltimore classification
- DNA viruses
- RNA viruses
- Reverse transcribing viruses
- Holmes classification
- LHT System of Virus Classification
- Subviral agents
- Viroids
- Satellites
- Prions
- Defective interfering particles
- References
Viruses are mainly classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. Currently, two main schemes are used for the classification of viruses: the International Committee on Taxonomy of Viruses (ICTV) system and Baltimore classification system, which places viruses into one of seven groups. Accompanying this broad method of classification are specific naming conventions and further classification guidelines set out by the ICTV.
A catalog of all the world's viruses has been proposed; some related preliminary efforts have been accomplished.
Virus species definition
Species form the basis for any biological classification system. The ICTV had adopted the principle that a virus species is a polythetic class of viruses that constitutes a replicating lineage and occupies a particular ecological niche. In July 2013, the ICTV definition of species changed to state: "A species is a monophyletic group of viruses whose properties can be distinguished from those of other species by multiple criteria."
ICTV classification
The International Committee on Taxonomy of Viruses began to devise and implement rules for the naming and classification of viruses early in the 1970s, an effort that continues to the present. The ICTV is the only body charged by the International Union of Microbiological Societies with the task of developing, refining, and maintaining a universal virus taxonomy.
The system shares many features with the classification system of cellular organisms, such as taxon structure. However, this system of nomenclature differs from other taxonomic codes on several points. A minor point is that names of orders and families are italicized, unlike in the International Code of Nomenclature for algae, fungi, and plants and International Code of Zoological Nomenclature.
Viral classification starts at the level of order and continues as follows, with the taxon suffixes given in italics:
Order (-virales) Family (-viridae) Subfamily (-virinae) Genus (-virus) SpeciesSpecies names generally take the form of [Disease] virus.
The establishment of an order is based on the inference that the virus families it contains have most likely evolved from a common ancestor. The majority of virus families remain unplaced. As of 2012, seven orders, 96 families, 22 subfamilies, 420 genera, and 2,618 species of viruses have been defined by the ICTV. The orders are the Caudovirales, Herpesvirales, Ligamenvirales, Mononegavirales, Nidovirales, Picornavirales, and Tymovirales. These orders span viruses with varying host ranges. The Ligamenvirales, infecting archaea, are the most recent addition to the classification system.
Caudovirales are tailed dsDNA (group I) bacteriophages.
Herpesvirales contain large eukaryotic dsDNA viruses.
Ligamenvirales contains linear, dsDNA (group I) archaean viruses.
Mononegavirales include nonsegmented (-) strand ssRNA (Group V) plant and animal viruses.
Nidovirales are composed of (+) strand ssRNA (Group IV) viruses with vertebrate hosts.
Picornavirales contains small (+) strand ssRNA viruses that infect a variety of plant, insect and animal hosts.
Tymovirales contain monopartite (+) ssRNA viruses that infect plants.
Other variations occur between the orders: Nidovirales, for example, are isolated for their differentiation in expressing structural and nonstructural proteins separately.
Structure-based virus classification
It has been suggested that similarity in virion assembly and structure observed for certain viral groups infecting hosts from different domains of life (e.g., bacterial tectiviruses and eukaryotic adenoviruses or prokaryotic Caudovirales and eukaryotic herpesviruses) reflects an evolutionary relationship between these viruses. Therefore, structural relationship between viruses has been suggested to be used as a basis for defining higher-level taxa - structure-based viral lineages - that could complement the existing ICTV classification scheme.
Baltimore classification
Baltimore classification (first defined in 1971) is a classification system that places viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), Sense, and method of replication. Named after David Baltimore, a Nobel Prize-winning biologist, these groups are designated by Roman numerals. Other classifications are determined by the disease caused by the virus or its morphology, neither of which are satisfactory due to different viruses either causing the same disease or looking very similar. In addition, viral structures are often difficult to determine under the microscope. Classifying viruses according to their genome means that those in a given category will all behave in a similar fashion, offering some indication of how to proceed with further research. Viruses can be placed in one of the seven following groups:
DNA viruses
RNA viruses
Reverse transcribing viruses
Holmes classification
Holmes (1948) used Carl Linnaeus's system of binomial nomenclature to classify viruses into 3 groups under one order, Virales. They are placed as follows:
LHT System of Virus Classification
The LHT System of Virus Classification is based on chemical and physical characters like nucleic acid (DNA or RNA), Symmetry (Helical or Icosahedral or Complex), presence of envelope, diameter of capsid, number of capsomers. This classification was approved by the Provisional Committee on Nomenclature of Virus (PNVC) of the International Association of Microbiological Societies (1962). It is as follows:
Subviral agents
The following agents are smaller than viruses but have only some of their properties.
Viroids
Satellites
Satellites depend on co-infection of a host cell with a helper virus for productive multiplication. Their nucleic acids have substantially distinct nucleotide sequences from either their helper virus or host. When a satellite subviral agent encodes the coat protein in which it is encapsulated, it is then called a satellite virus.
Prions
Prions, named for their description as "proteinaceous and infectious particles", lack any detectable (as of 2002) nucleic acids or virus-like particles. They resist inactivation procedures that normally affect nucleic acids.